Traditional flexographic printing methods prepare a printing plate (or a printing cylinder) by molding an elastomer, such as rubber, in a mold, or by photo-polymerizing a UV sensitive polymer. These methods are slow and expensive.
While it would be highly desirable to create flexographic printing plates in the form of a seamless sleeve it is generally impractical to do so because conventional flexographic printing surfaces, such as photo-polymer plates, typically require some chemical processing. Chemical processing is impractical for seamless sleeves and is much easier to perform on flat plates.
Another technique for creating a raised pattern on an elastomer is to directly cut the raised pattern using a CO.sub.2 laser. The laser is controlled to ablate the elastomer in recessed areas and to leave the elastomer intact in raised areas. Direct laser processing is advantageous because it does not require any chemical processing or other intermediate process steps. As the data to be imaged is available in electronic form, it would appear that going directly from digital data to a CO.sub.2 laser based engraver would be the most accurate and efficient way for making flexographic printing plates.
Conventional flexographic printing cannot be laser engraved quickly. This is because the laser must ablate a relatively thick layer (0.5 mm-2 mm) of elastomer. Further, typical elastomer materials as used in flexographic printing plates have ablation rates of only about 0.3 mm.sup.3 /w/sec. Thus a multi-KW laser is required to complete the task of engraving a typical flexographic plate in under one hour. Another difficulty with previous attempts at laser engraving of flexographic printing surfaces which use CO.sub.2 lasers is that CO.sub.2 lasers have a long wavelength (10.6 microns) which severely limits the resolution that can be achieved. The best resolution achievable with a laser is proportional to the wavelength of the laser.
Evans, U.S. Pat. No. 4,060,032 discloses a multi-layer flexographic printing plate which includes a metallic writing layer, a barrier layer, and a polymer substrate layered atop a metal backing. The polymer substrate is cellular so that its density is reduced in comparison to a solid polymer. The reduced density substrate can be laser ablated more quickly than a denser material. The Evans printing plate is developed in a two step process. First, a visible laser, such as an argon laser, is used to remove the metallic writing in portions of the plate which should be recessed to form a mask. Then an infrared laser, such as a CO.sub.2 laser, is used to remove the barrier layer and a portion of the substrate layer in the areas exposed by the mask. The writing layer reflects the infrared laser beam in other areas.
The Evans methods and printing plates has three significant disadvantages. First the plates themselves are undesirably complicated to make as they have several layers including a top metallic mask layer. Second, there is a trend toward the use of thinner backings and thinner elastomeric layers in flexographic printing plates. The Evans methods can result in localized damage to thin backings if the CO.sub.2 laser is allowed to ablate away all of the substrate layer in any location. A CO.sub.2 laser sufficiently powerful to ablate the polymer layer in an Evans printing plate is capable of damaging thin backings. Thirdly, a CO.sub.2 laser is typically incapable of achieving a resolution sufficient for making a printing plate. The Evans method is limited to creating plates in a two part process in which a high resolution mask is formed with a first laser and then the barrier layer and substrate are removed using a lower resolution CO.sub.2 laser.
Barker, U.S. Pat. No. 3,832,948 discloses another method for making a printing plate. Like the method of Evans, the Barker method requires two separate laser ablation steps to create a printing plate.
Shuji, U.S. Pat. No. 4,943,467 discloses a plate for use in printing on corrugated board. The Shuji printing plate has a smooth skin layer disposed atop a foam layer. The smooth skin layer is quite thick, being in the range of 0.3 mm thick to 2.0 mm thick. The alleged advantage of the Shuji et al plate is that printing pressure can be reduced, thereby reducing damage to the corrugated board being imprinted. The Shuji et al plates are sculpted by mechanically cutting away the skin layer and the foam layer in recessed areas.
There remains a need for a method for direct laser imprinting flexographic printing plates which avoids the disadvantages set out above. There is particular need for a method for the direct laser imprinting of flexographic printing plates provided as seamless sleeves.